Adaptive solar modulation of thermochromic VO2 toward high-performance energy-efficient smart windows

• Main Author: Ke, Yujie
• Other Authors: Timothy John White
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Materials::Functional materials; DRNTU::Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/104819; http://hdl.handle.net/10220/48089
• Institution: Nanyang Technological University

Smart windows are promised with a significant contribution to the economization of building energy consumption. Such windows can dynamically modulate the indoor solar irradiation, leading to energy-saving for architectural heating and cooling systems. Development of smart windows can mainly be categorized as electro-, thermo-, and photo-stimulated optical response changes. Among them, thermochromic smart windows that change the optical properties in responsive to the temperature are highly competitive due to the low cost, passivity, and rational stimulus-response. Vanadium dioxide (VO2) is a promising candidate for thermochromic smart window application due to its reversible phase transition near the room temperature, accompanied with huge optical properties change. It can be transparent to the near-infrared solar irradiation at low temperature, while shield it at high temperature. However, challenges remain in VO2-based thermochromic smart windows, including the intrinsic yellow-brown color, the relatively low solar energy modulation (ΔTsol) and visible transmittance (Tlum). This thesis is dedicated to improve these shortages and promote the performance of VO2-based thermochromic smart windows, resulting in three effective methods. The photonic crystals were introduced to improve the undesirable yellow-brown color. The Silica/VO2 (SiO2/VO2) core/shell two-dimensional (2D) photonic crystal is prepared and demonstrated to exhibit static visible light tunability and dynamic near-infrared (NIR)modulation. Three-dimensional finite difference time domain simulations predict that the transmittance can be tuned across the visible spectrum while maintaining good ΔTsol of 11.0% and a Tlum of 49.6%. Though the experimental result is not as prominent, it proves that the color changes of VO2 films are accompanied by NIR modulation, which is aligned with the simulation result. In produced samples, colors beyond the yellow-brown are observed, such as green, blue, and red, demonstrating the effectiveness of photonic crystals for tuning the color of VO2-based smart windows. The localized surface plasmon resonance (LSPR) was introduced to promote the performance of the ΔTsol and the Tlum. The phenomenon is demonstrated in tuning the absorption peak position of VO2 to the region with dense solar energy, which was demonstrated on 2D patterned VO2 nanoparticles. Colloidal lithography method was applied to prepare the 2D patterned VO2-based nanocrystals, resulting in nanonet, nanodome, to nanoparticle arrays. LSPR and their application in thermochromic smart windows. The nanoparticle arrays exhibit tunable LSPR at different temperatures and the LSPR red-shift is observed with the increase of the particle size and the media reflective index. A competitive solar regulation efficiency (ΔTsol = 13.2%) and a transmittance contrast at 2000 nm (ΔTNIR = 44%) can be achieved. Pairing reconfigurable metamaterials with the active LSPR was demonstrated to provide performance. A kirigami-inspired elastomer containing plasmonic vanadium dioxide was developed, exhibiting adaptive, broadband, and high-efficient optical modulation. The geometrical transition and the active LSPR function synergistically by manipulating ultraviolet-visible and near-infrared regions, respectively. A new mechanical-thermal energy-saving smart window was developed adopting such materials exhibits an unprecedented solar energy modulation (ΔTsol = 37.7%). Fully thermal responsiveness is also achieved in such a system. Smart window based on such materials are promised better energy-saving ability than commercialized low-emissivity glasses in cities, such as Hong Kong and Houston.